irradiated by internal sources; that 1s, by radionuclides incorporated in body tissues. These radionuclides gain entrance inhalation or inte the boly threugi taminated feod or water. through con- Once inside they behave like their non-radioactive counterparts. Radioactive iodine, for exainple, accumulates in the thyroid gland in the same fashion as stable iodine, | and radioactive strontium or calcium accumulate in the bone similar to their naturally occurring non-radio- active counterparts. The radioactive iodine will thus deliver a dosage to the thyroid gland that is many times larger than that to the other organs or to the whole body, and the radioactive strontium and calcium will mainly irradiate the bone. Because of the uneven distribution of radionuclides in the body organs, radiation exposure standards have been developed not just for the whole body, but also for individual organs. In this report we will be referrjng to the maximum permissible whole body and lung doses. Largely as a matter of convenience, radiation standards have been developed. secondary or derived These secondary standards, which limit radionuclide concentrations or organ burdens, are often more easily employed than the primary dose standards. We shall examine two secondary standards in this le